1. Field of the Invention
This invention relates to an image forming apparatus such as a printer, a copier, a facsimile, and so on, which forms an image on a record medium based on bit map data.
2. Discussion of the Background
An image forming apparatus, for example a printer, records an image at high speed page by page. Therefore, in a printer, at first, bit map data of one or more pages is stored in a frame memory. Thereafter, the bit map data which is stored in the frame memory is read continuously and a printer records an image based on this bit map data.
However, because a quantity of the bit map data is large, a large frame memory is needed, and as a result the cost of the printer increases. To prevent this cost increase, a printer which has a frame memory of a smaller capacity is known. In this printer, bit map data is compressed and is stored in the smaller capacity frame memory, and when the stored bit map data is read out of this frame memory, the bit map data is decompressed.
However, this printer which stores bit map data in the frame memory with a compression operation has the following problems.
First, when the compression is an non-reversible compression, which obtains a high compression ratio, an image which is formed on a record medium is degraded because the bit map data of the original image is not exactly reproduced. This image degradation is not conspicuous in an image area but is conspicuous in a text area.
A second problem is concerned with a speed follow-ability. An overrun is a phenomenon in which a speed of a printing operation exceeds that of a decompression operation, resulting in print data not being supplied to the printer when the printer is ready to print data.
To solve the first problem, an operation of separating a text area and an image area in a recording apparatus which executes a compression operation at a compression ratio which is suited for each area is proposed by Japanese Patent Laid-Open No. HEI 4-282271. In this apparatus, first it is determined whether data stored in a page buffer is image data or text data. Then, based on a distinction of this determination, when the data is determined to be text data, the data is compressed at a compression ratio which is suited to text data by a text data compression circuit. And, when the data is determined to be image data, the data is compressed at a compression ratio which is suited to image data by an image data compression circuit. Then, the compressed text data is stored in a frame memory for text data, and also the compressed image data is stored in a frame memory for image data. The stored text data and the stored image data are then decompressed by a decompression circuit which corresponds to each type of data when read out. The decompressed data is then recorded onto a record medium.
On the other hand, to solve the second problem, an image forming apparatus which makes an operation speed of a printer low-speed based on the decompressed image data is proposed by Japanese Patent Laid-Open No. HEI 6-183070. In this apparatus, first it is determined whether given data is bit image data, character code data or compression image data. Based on this determination, when the given data is determined to be compression image data, a print speed of a printer is changed to a low speed as required by the time necessary for a decompression operation to be executed. This apparatus tries to prevent an overrun condition by making a decompression speed and print speed agree by changing the print speed.
However, in the background art described Japanese Patent Laid-Open No. HEI 4-282271, depending on the page-description language which is input to the apparatus, the determination between the text data and an image data is sometimes difficult. In this case, there is a drawback that degradation of the image formed on the record medium may occur.
Also, in the background art described Japanese Patent Laid-Open No. HEI 6-183070, the cost of the apparatus rises because a unit for changing a print speed is necessary. Moreover, a speed of a printer has a lower limit from a characteristic of an apparatus. For these reasons, there is a drawback that an overrun condition may still occur in the case that a decompression speed is very low.